1. Field of the Invention
The present invention relates to the margin at the leading edge of a recording medium on which an image is printed by an electrophotographic apparatus such as a copier, printer, or facsimile machine.
2. Description of the Related Art
In conventional electrophotographic printing apparatus, an image formed with a developing agent such as toner is transferred to a recording medium such as a sheet of paper, which is then passed between a fusing roller and a pressure roller that fuse the image onto the recording medium by a combination of heat and pressure. Since heating increases the adhesiveness of the developing agent, the recording medium has a tendency to stick to the fusing roller and must be separated therefrom by a claw or some other type of separator.
The separator must not make contact with the surface of the fusing roller, lest it damage the surface, so a gap is left between the separator and the fusing roller. As the recording medium approaches the separator, accordingly, the leading edge of the recording medium must separate naturally from the fusing roller so that it can engage the separator. A certain margin of space must therefore be left free of developing agent at the leading edge of the recording medium. This causes a problem when the image to be printed extends into the margin.
The necessary length of the margin as measured from the leading edge of the recording medium in the direction of travel of the recording medium is related to the density of the developing agent on the recording medium, which depends both on the content of the image and the type of image processing apparatus. The density is higher in color image forming apparatus, in which toner developing agents of different colors are superimposed on the recording medium, than in a monochrome printer. The higher the density of the recording agent, the more the recording media tend to stick to the fusing roller. In color image forming apparatus the necessary length of the non-printable margin at the leading edge of the recording medium is generally considered to be at least five millimeters (5 mm).
The user of the image forming apparatus would naturally prefer the non-printable margin to be as small as possible. A reduced margin, however, carries with it the risk that recording media will fail to separate from the fusing roller and will jam. Since this risk varies with the density of the developing agent near the margin, Japanese Unexamined Patent Application Number No. 2000-47515 discloses an image forming apparatus that switches from a relatively small margin to a larger margin when, for example, the developing agent density in the first centimeter, as measured from the leading edge, is greater than a predetermined threshold.
This prior art, however, fails to take cognizance of other factors that affect the risk of non-separation and jamming. In particular, thin recording media have a greater tendency to stick to the fusing roller than do thick recording media, because thick recording media are stiffer than thin recording media. The orientation of the media is also a factor: recording media transported long edge first have a greater tendency to stick to the fusing roller than recording media transported short edge first. Setting the non-printable margin solely according to the developing agent density leads to the setting of an unnecessarily large margin for thick recording media, and for recording media transported short edge first.
A further shortcoming of this prior art is that it always leaves at least a small non-printable margin, regardless of the density of the developing agent. Marginless printing, which is often desirable for photographic images and the like, is not possible.